Use of a permanent magnet synchronous generator in the power train of a wind turbine has some advantages compared to other types of generators more commonly used on wind turbines. These advantages include, among others things, high efficiency, lower losses on the rotor of the generator (which is a difficult part to cool down), lack of any slip-rings and their attendant problems, and better low voltage ride through performance.
However, when a short circuit event, such as an arc, occurs in the generator or converter during normal operation, a permanent magnet synchronous generator will continue to generate stator voltage as long as the generator is turning since the permanent magnets in the rotor will continue to produce a magnetic field. Therefore, short circuit current is produced under such conditions and must be properly handled to avoid damage to the wind turbine.
Different types of short circuit events can occur and may be handled in different ways to reduce impact on the wind turbine. For instance, in a polyphase generator and power converter configuration, a power converter failure will in most cases appear as a three-phase short circuit on the generator. A three-phase short circuit on the generator causes a transient torque oscillation followed by an almost complete unloading of the drive train. Such an unloading causes undesirable mechanical load levels on the wind turbine. To reduce the impact from such a short circuit event, the wind turbine may have means to disconnect the converter from the generator and for subsequently applying a passive dump load, which will serve to reintroduce a certain load torque on the drive train.
Another short circuit event, which appears somewhat rarely, is a three-phase short circuit inside a three-phase generator. This kind of short circuit event also causes a transient torque oscillation followed by an almost complete unloading of the drive train. From an electrical point of view, no measures, such as application of a passive dump load, can be taken to counteract this undesired unloading of the drive train.
A third type of short circuit event is an unbalanced short circuit in the generator of a wind turbine. When, for example, a two-phase short circuit (which is a type of unbalanced short circuit) occurs in the generator, a transient torque oscillation appears followed by a stationary torque oscillation with a frequency content determined by the speed of the generator. One countermeasure that can be taken in response to an unbalanced short circuit in the generator is to shut down the wind turbine (i.e., bring the wind turbine to a standstill). However, as shown, for example, in FIGS. 8, 9, and 10, the drive train oscillation would go through a very large frequency range resulting in a high risk of exciting resonances in the drive train and turbine foundation. Moreover, as with the three-phase short circuit event, application of a dump load would have no substantial effect on the torque oscillations.
FIG. 7 shows a controlled speed ramp down of the generator in a wind turbine when it is shut down due to a two-phase short circuit in a generator winding. FIG. 8 shows a graph of an example torque response of the generator associated with the speed ramp down profile in FIG. 7, but without application of the dump load. The graph includes two curves, a first one calculated by a dynamic model, and a second one calculated by a simple analytical approach. FIGS. 9 and 10 show different zoomed in views of the graph in FIG. 8. As shown in FIGS. 9 and 10, the torque oscillation contains a wide range of harmonics.
FIG. 11 shows a graph of an example torque response of the generator associated with the speed ramp down profile shown in FIG. 1, but in this graph the dump load has been applied. FIGS. 12 and 13 show different zoomed in views of the graph in FIG. 11. As shown in FIGS. 12 and 13, the torque oscillations contain a wide range of harmonics regardless of the fact that a dump load has been applied. Thus, the torque oscillations due to a two-phase short circuit are just as severe as without the dump load.
“IPM Synchronous Machine Drive Response to Symmetrical and Asymmetrical Short Circuit Faults” by Welchko et al. (IEEE Transactions on Energy Conversion, Vol. 18, No. 2, June 2003) describes long-lasting, high amplitude torque oscillations applied to an interior permanent magnet (IPM) synchronous machine due to failure of an inverter switch of an adjustable-speed drive that supplies current to the IPM machine. The failure of the inverter switch results in an asymmetrical short circuit at the terminals of the IPM machine. Welchko et al. propose closing additional switches in the adjustable-speed drive to create a balanced short circuit fault, which reduces the potentially damaging levels of torque oscillations. However, this approach has the drawback of introducing high levels of current into components of the adjustable-speed drive that may not be rated for high currents. The same problem occurs when the IPM machine is used as a generator rather than a motor and a converter is used instead of the adjustable-speed drive.
A short circuit or short circuit event is frequently referred to herein as occurring “in” a generator. However, the location of an unintended electrically conductive connection (e.g., an arc) that causes a short circuit to occur in the generator may be external to a housing of the generator. For example, an unintended electrical connection may occur at any number of places external to the generator housing (e.g., between cables carrying power produced by the generator to a converter, between electrical conductors within the converter, between conductors in a complex cable termination or in a device, such as a circuit breaker or voltage protection device) but can have substantially the same short circuit effect as an unintended electrical connection that occurs internal to the generator housing. Thus, an unintended electrical connection that occurs either inside or outside of the generator housing creates a short circuit in the generator if one or more conductive elements in the generator are in an electrically conductive circuit (i.e., a short circuit) that includes the unintended electrical connection.